Sound-processing strategy for cochlear implants

ABSTRACT

A sound processing method for auditory prostheses, such as cochlear implants, which is adapted to improve the perception of loudness by users, and to improve speech perception. The overall contribution of stimuli to simulated loudness is compared with an estimate of acoustic loudness for a normally hearing listener based on the input sound signal. A weighting is applied to the filter channels to emphasize those frequencies which are most important to speech perception for normal hearing listeners when selecting channels as a basis for stimulation.

TECHNICAL FIELD

[0001] This invention relates to pulsatile type multi-channel cochlearimplant systems for the totally or profoundly deaf.

BACKGROUND OF THE INVENTION

[0002] Pulsatile multi-channel cochlear implant systems generallyinclude a cochlear implant, an external speech processor, and anexternal headset. The cochlear implant delivers electrical stimulationpulses to an electrode array (e.g., 22 electrodes) placed in thecochlea. The speech processor and headset transmit information and powerto the cochlear implant.

[0003] The speech processor operates by receiving an incoming acousticsignal from a microphone in the headset, or from an alternative source,and extracting from this signal specific acoustic parameters. Thoseacoustic parameters are used to determine electrical stimulationparameters, which are encoded and transmitted to the cochlear implantvia a transmitting coil in the headset, and a receiving coil formingpart of the implant.

[0004] In many people who are profoundly deaf, the reason for deafnessis absence of, or destruction of, the hair cells in the cochlea whichtransduce acoustic signals into nerve impulses. These people are thusunable to derive any benefit from conventional hearing aid systems, nomatter how loud the acoustic stimulus is made, because it is notpossible for nerve impulses to be generated from sound in the normalmanner. Cochlear implant systems seeks to bypass these hair cells bypresenting electrical stimulation to the auditory nerve fibers directly,leading to the perception of sound in the brain. There have been manyways described in the past for achieving this object, running fromimplantation of electrodes in the cochlea connected to the outside worldvia a cable and connector attached to the patient's skull, tosophisticated multichannel devices communicating with an externalcomputer via radio frequency power and data links.

[0005] The invention described herein is particularly suited for use ina prosthesis which comprises a multichannel electrode implanted into thecochlea, connected to a multichannel implanted stimulator unit, whichreceives power and data from an externally powered wearable speechprocessor, wherein the speech processing strategy is based on knownpsychophysical phenomenon, and is customized to each individual patientby use of a diagnostic and programming unit. One example of such aprosthesis is the one shown and described in U.S. Pat. No. 4,532,930 toCrosby et al., entitled “Cochlear Implant System for an AuditoryProsthesis.”

[0006] In order to best understand the invention it is necessary to beaware of some of the physiology and anatomy of human hearing, and tohave a knowledge of the characteristics of the speech signal. Inaddition, since the hearing sensations elicited by electricalstimulation are different from those produced by acoustic stimulation ina normal hearing person, it is necessary to discuss the psychophysics ofelectrical stimulation of the auditory system. In a normal hearingperson, sound impinges on the ear drum, as illustrated in FIG. 1, and istransmitted via a system of bones called the ossicles, which act aslevers to provide amplification and acoustic impedance matching to apiston, or membrane, called the oval window, which is coupled to thecochlea chamber.

[0007] The cochlear chamber is about 35 mm long when unrolled and isdivided along most of its length by a partition. This partition iscalled the basilar membrane. The lower chamber is called the scalatympani. An opening at the remote end of the cochlea chambercommunicates between the upper and lower halves thereof. The cochlea isfilled with a fluid having a viscosity of about twice that of water. Thescala tympani is provided with another piston or membrane called theround window which serves to take up the displacement of the fluid.

[0008] When the oval window is acoustically driven via the ossicles, thebasilar membrane is displaced by the movement of fluid in the cochlea.By the nature of its mechanical properties, the basilar membranevibrates maximally at the remote end or apex of the cochlea for lowfrequencies, and near the base or oval window thereof for highfrequencies. The displacement of the basilar membrane stimulates acollection of cells called the hair cells situated in a specialstructure on the basilar membrane. Movements of these hairs produceelectrical discharges in fibers of the VIIIth nerve, or auditory nerve.Thus the nerve fibers from hair cells closest to the round window (thebasal end of the cochlea) convey information about high frequency sound,and fibers more apical convey information about low frequency sound.This is referred to as the tonotopic organization of nerve fibers in thecochlea.

[0009] Hearing loss may be due to many causes, and is generally of twotypes. Conductive hearing loss occurs when the normal mechanicalpathways for sound to reach the hair cells in the cochlea are impeded,for example by damage to the ossicles. Conductive hearing loss may oftenbe helped by use of hearing aids, which amplify sound so that acousticinformation does reach the cochlea. Some types of conductive hearingloss are also amenable to alleviation by surgical procedures.

[0010] Sensorineural hearing loss results from damage to the hair cellsor nerve fibers in the cochlea. For this type of patient, conventionalhearing aids will offer no improvement because the mechanisms fortransducing sound energy into nerve impulses have been damaged. It is bydirectly stimulating the auditory nerve that this loss of function canbe partially restored.

[0011] In the system described herein, and in some other cochlearimplant systems in the prior art, the stimulating electrodes aresurgically placed in the scala tympani, in close proximity to thebasilar membrane, and currents that are passed between the electrodesresult in neural stimulation in groups of nerve fibers.

[0012] The human speech production system consists of a number ofresonant cavities, the oral and the nasal cavities, which may be excitedby air passing through the glottis or vocal cords, causing them tovibrate. The rate of vibration is heard as the pitch of the speaker'svoice and varies between about 100 and 400 Hz. The pitch of femalespeakers is generally higher than that of male speakers.

[0013] It is the pitch of the human voice which gives a sentenceintonation, enabling the listener, for instance, to be able todistinguish between a statement and a question, segregate the sentencesin continuous discourse and detect which parts are particularlystressed. This together with the amplitude of the signal provides theso-called prosodic information.

[0014] Speech is produced by the speaker exciting the vocal cords, andmanipulating the acoustic cavities by movement of the tongue, lips andjaw to produce different sounds. Some sounds are produced with the vocalcords excited, and these are called voiced sounds. Other sounds areproduced by other means, such as the passage of air between teeth andtongue, to produce unvoiced sounds. Thus the sound “Z” is a voicedsound, whereas “s” is an unvoiced sound; “B” is a voiced sound, and “P”is an unvoiced sound, etc.

[0015] The speech signal can be analyzed in several ways. One usefulanalysis technique is spectral analysis, whereby the speech signal isanalyzed in the frequency domain, and a spectrum is considered ofamplitude (and phase) versus frequency. When the cavities of the speechproduction system are excited, a number of spectral peaks are produced,and the frequencies and relative amplitudes of these spectral peaks arealso varied with time.

[0016] The number of spectral peaks ranges between about three and fiveand these peaks are called “formants”. These formants are numbered fromthe lowest frequency formant, conventionally called F1, to the highestfrequency formants, and the voice pitch is conventionally referred to asF0. Characteristic sounds of different vowels are produced by thespeaker changing the shape of the oral and nasal cavities, which has theeffect of changing the frequencies and relative intensities of theseformants.

[0017] In particular, it has been found that the second formant (F2) isimportant for conveying vowel information. For example, the vowel sounds“oo” and “ee” may be produced with identical voicings of the vocalcords, but will sound different due to different second formantcharacteristics.

[0018] There is of course a variety of different sounds in speech andtheir method of production is complex. For the purpose of understandingthe invention herein however, it is sufficient to remember that thereare two main types of sounds—voiced and unvoiced; and that the timecourse of the frequencies and amplitudes of the formants carries most ofthe intelligibility of the speech signal.

[0019] The term “psychophysics” is used herein to refer to the study ofthe perceptions elicited in patient's by electrical stimulation of theauditory nerve. For stimulation at rates between 100 and 400 pulses persecond, a noise is perceived which changes pitch with stimulation rate.This is such a distinct sensation that it is possible to convey a melodyto a patient by its variation.

[0020] By stimulating the electrode at a rate proportional to voicepitch (F0), it is possible to convey prosodic information to thepatient. This idea is used by some cochlear implant systems as the solemethod of information transmission, and may be performed with a singleelectrode.

[0021] It is more important to convey formant information to thepatient, as this contains most of the intelligibility of the speechsignal. It has been discovered by psychophysical testing that just as anauditory signal which stimulates the remote end of the cochlea producesa low frequency sensation and a signal which stimulates the near endthereof produces a high frequency sensation, a similar phenomenon willbe observed with electrical stimulation. The perceptions elicited byelectrical stimulation at different positions inside the cochlea havebeen reported by the subjects as producing percepts which vary in“sharpness” or “dullness”, rather than pitch as such. However, thedifference in frequency perceptions between electrodes is such thatformant, or spectral peak, information can be coded by selection ofelectrode, or site of stimulation in the cochlea.

[0022] It has been found by psychophysical testing that the perceivedloudness of sounds elicited by electrical stimulation of the auditorynerve has a larger dynamic range then the dynamic range of thestimulation itself. For example, a 220 dB dynamic range of electricalstimulation may produce perceptions from threshold or barelyperceivable, to threshold of pain. In normal hearing people the dynamicrange of sound perception is in the order of 100 dB.

[0023] It has also been discovered through psychophysical testing thatthe pitch of sound perceptions due to electrical stimulation is alsodependent upon frequency of stimulation, but the perceived pitch is notthe same as the stimulation frequency. In particular, the highest pitchable to be perceived through the mechanism of the changing stimulationrate alone is in the order of 1 kHz, and stimulation at rates above thismaximum level will not produce any increase in frequency or pitch of theperceived sound. In addition, for electrical stimulation within thecochlea, the perceived pitch depends upon electrode position. Inmultiple electrode systems, the perceptions due to stimulation at oneelectrode are not independent of the perceptions due to simultaneousstimulation of nearby electrodes. Also, the perceptual qualities ofpitch, “sharpness”, and loudness are not independently variable withstimulation rate, electrode position, and stimulation amplitude.

[0024] Some systems of cochlear implants in the prior art are arrangedto stimulate a number of electrodes simultaneously in proportion to theenergy in specific frequency bands, but this is done without referenceto the perceptions due to stimulus current in nearby stimulatingelectrodes. The result is that there is interaction between the channelsand the loudness is affected by this.

[0025] A number of attempts have heretofore been made to provide usefulhearing through electrical stimulation of auditory nerve fibers, usingelectrodes inside or adjacent to some part of the cochlear structure.Systems using a single pair of electrodes are shown in U.S. Pat. No.3,751,605 to Michelson and U.S. Pat. No. 3,752,939 to Bartz.

[0026] In each of these systems an external speech processing unitconverts the acoustic input into a signal suitable for transmissionthrough the skin to an implanted receiver/stimulator unit. These devicesapply a continuously varying stimulus to the pair of electrodes,stimulating at least part of the population of auditory nerve fibers,and thus producing a hearing sensation.

[0027] The stimulus signal generating from a given acoustic input isdifferent for each of these systems, and while some degree ofeffectiveness has been demonstrated for each, performance has variedwidely across systems and also for each system between patients. Becausethe design of these systems has evolved empirically, and has not beenbased on detailed psychophysical observations, it has not been possibleto determine the cause of this variability. Consequently, it has notbeen possible to reduce it.

[0028] An alternative approach has been to utilize the tonotopicorganization of the cochlea to stimulate groups of nerve fibers,depending on the frequency spectrum of the acoustic signal. Systemsusing this technique are shown in U.S. Pat. No. 4,207,441 to Ricard,U.S. Pat. No. 3,449,753 to Doyle, U.S. Pat. No. 4,063,048 to Kissiah,and U.S. Pat. No. 4,284,856 and No. 4,357,497 to Hochmair et al.

[0029] The system described by Kissiah uses a set of analog filters toseparate the acoustic signal into a number of frequency components, eachhaving a predetermined frequency range within the audio spectrum. Theseanalog signals are converted into digital pulse signals having a pulserate equal the frequency of the analog signal they represent, and thedigital signals are used to stimulate the portion of the auditory nervenormally carrying the information in the same frequency range.Stimulation is accomplished by placing an array of spaced electrodesinside the cochlea.

[0030] The Kissiah system utilizes electrical stimulation at rates up tothe limit of normal acoustic frequency range, say 10 kHz, andindependent operation of each electrode. Since the maximum rate offiring of any nerve fiber is limited by physiological mechanisms to oneor two kHz, and there is little perceptual difference for electricalpulse rates above 800 Hz, it may be inappropriate to stimulate at therates suggested. No consideration is given to the interaction betweenthe stimulus currents generated by different electrodes, whichexperience shows may cause considerable uncontrolled loudnessvariations, depending on the relative timing of stimulus presentations.Also, this system incorporates a percutaneous connector which has withit the associated risk of infection.

[0031] The system proposed by Doyle limits the stimulation rate for anygroup of fibers to a rate which would allow any fiber to respond tosequential stimuli. It utilizes a plurality of transmission channels,with each channel sending a simple composite power/data signal to abipolar pair of electrodes. Voltage source stimulation is used in a timemultiplexed fashion similar to that subsequently used by Ricard anddescribed below, and similar uncontrolled loudness variations will occurwith the suggested independent stimulation of neighboring pairs ofelectrodes. Further, the requirement of a number of transmission linksequal to the number of electrode pairs prohibits the use of this type ofsystem for more than a few electrodes.

[0032] The system proposed by Ricard utilizes a filter bank to analyzethe acoustic signal, and a single radio link to transfer both power anddata to the implanted receiver/stimulator, which presents a timemultiplexed output to sets of electrodes implanted in the cochlea.Monophasic voltage stimuli are used, with one electrode at a time beingconnected to a voltage source while the rest are connected to a commonground line. An attempt is made to isolate stimulus currents from oneanother by placing small pieces of silastic inside the scala, betweenelectrodes. Since monophasic voltage stimuli are used, and theelectrodes are returned to the common reference level after presentationof each stimulus, the capacitive nature of the electrode/electrolyteinterface will cause some current to flow for a few hundred microsecondsafter the driving voltage has been returned to zero. This will reducethe net transfer of charge (and thus electrode corrosion) but thischarge recovery phase is now temporarily overlapped with the followingstimulus or stimuli. Any spatial overlap of these stimuli would thencause uncontrolled loudness variations.

[0033] In the Hochmair et al. patents a plurality of carrier signals aremodulated by pulses corresponding to signals in audio frequency bands.The carrier signals are transmitted to a receiver having independentchannels for receiving and demodulating the transmitted signals. Thedetected pulses are applied to electrodes on a cochlear implant, withthe electrodes selectively positioned in the cochlea to stimulateregions having a desired frequency response. The pulses have a frequencywhich corresponds to the frequency of signals in an audio band and apulse width which corresponds to the amplitude of signals in the audioband.

[0034] U.S. Pat. No. 4,267,410 to Forster et al. describes a systemwhich utilizes biphasic current stimuli of predetermined duration,providing a good temporal control of both stimulating and recoveryphases. However, the use of fixed pulse duration prohibits variation ofthis parameter which may be required by physiological variations betweenpatients. Further, the data transmission system described in this systemseverely limits the number of pulse rates available for constant ratestimulation.

[0035] U.S. Pat. No. 4,593,696 to Hochmair et al. describes a system inwhich at least one analog electrical signal is applied to implantedelectrodes in a patient, and at least one pulsatile signal is applied toimplanted electrodes. The analog signal represents a speech signal, andthe pulsatile signal provides specific speech features such as formantfrequency and pitch frequency.

[0036] U.S. Pat. No. 4,515,158 to Patrick et al. describes a system inwhich sets of electrical currents are applied to selected electrodes inan implanted electrode array. An incoming speech signal is processed togenerate an electrical input corresponding to the received speechsignal, and electrical signals characterizing accoustic features of thespeech signal are generated from the input signal. Programmable meansobtains and stores data from the electrical signals and establishes setsof electric stimuli to be applied to the electrode array, andinstruction signals are produced for controlling the sequentialapplication of pulse stimuli to the electrodes at a rate derived fromthe voicing frequency of the speech signal for voiced utterances and ata rate independent of the voicing frequency for unvoiced utterances.

[0037] The state of the art over which the present invention representsan improvement is perhaps best exemplified by the aforesaid U.S. Pat.No. 4,532,930 to Crosby et al., entitled “Cochlear Implant System for anAuditory Prosthesis”. The subject matter of said Crosby et al. patent ishereby incorporated herein by reference. The Crosby et al. patentdescribes a cochlear implant system in which an electrode arraycomprising multiple platinum ring electrodes in a silastic carrier isimplanted in the cochlea of the ear. The electrode array is connected toa multi-channel receiver-stimulating unit, containing a semiconductorintegrated circuit and other components, which is implanted in thepatient adjacent the ear. The receiver-stimulator unit receives datainformation and power through a tuned coil via an inductive link with apatient-wearable external speech processor. The speech processorincludes an integrated circuit and various components which areconfigured or mapped to emit data signals from an Erasable ProgrammableRead Only Memory (EPROM). The EPROM is programmed to suit each patient'selectrical stimulation perceptions, which are determined through testingof the patient and his implanted stimulator/electrode. The testing isperformed using a diagnostic and programming unit (DPU) that isconnected to the speech processor by an interface unit.

[0038] The Crosby et al. system allows use of various speech processingstrategies, including dominant spectral peak and amplitude compressionof voice pitch, so as to include voiced sounds, unvoiced glottal soundsand prosodic information. The speech processing strategy employed isbased on known psychophysical phenomenon, and is customized to eachindividual patient by the use of the diagnostic and programming unitBiphasic pulses are supplied to various combinations of the electrodesby a switch controlled current sink in various modes of operation.Transmission of data is by a series of discrete data bursts whichrepresent the chosen electrode(s), the electrode mode configuration, thestimulating current, and amplitude determined by the duration of theamplitude burst.

[0039] Each patient will have different perceptions resulting fromelectrical stimulation of the cochlea. In particular, the strength ofstimulation required to elicit auditory perceptions of the same loudnessmay be different from patient to patient, and from electrode toelectrode for the same patient. Patients also may differ in theirabilities to perceive pitch changes from electrode to electrode.

[0040] The speech processor accommodates differences in psychophysicalperceptions between patients and compensates for the differences betweenelectrodes in the same patient. Taking into account each individual'spsychophysical responses, the speech processor encodes acousticinformation with respect to stimulation levels, electrode frequencyboundaries, and other parameters that will evoke appropriate auditoryperceptions. The psychophysical information used to determine suchstimulation parameters from acoustic signals is referred to as a MAP andis stored in a random access memory (RAM) inside the speech processor.An audiologist generates and “fine tunes” each patient's MAP using adiagnostic and programming system (DPS). The DPS is used to administerappropriate tests, present controlled stimuli, and confirm and recordtest results.

[0041] The multi-electrode cochlear prosthesis has been usedsuccessfully by profoundly deaf patients for a number of years and is apart of everyday life for many people in various countries around theworld. The implanted part of the prosthesis has remained relativelyunchanged except for design changes, such as those made to reduce theoverall thickness of the device and to incorporate an implanted magnetto eliminate the need for wire headsets.

[0042] The external speech processor has undergone significant changessince early versions of the prosthesis. The speech coding scheme used byearly patients presented three acoustic features of speech to implantusers. These were amplitude, presented as current level of electricalstimulation; fundamental frequency or voice pitch, presented as rate ofpulsatile stimulation; and the second formant frequency, represented bythe position of the stimulating electrode pair. This coding scheme(F0F2) provided enough information for profoundly postlinguisticallydeafened adults to show substantial improvements in their perception ofspeech.

[0043] The early coding scheme progressed naturally to a later codingscheme in which additional spectral information is presented. In thisscheme a second stimulating electrode pair was added, representing thefirst formant of speech. The new scheme (F0F1F2) showed improvedperformance for adult patients in all areas of speech perception.

[0044] Despite success of speech processors using the F0F1F2 scheme overthe last few years, a number of problems have been identified. Forexample, patients who perform well in quiet conditions can havesignificant problems when there is a moderate level of background noise.Also, the F0F1F2 scheme codes frequencies up to about 3,500 Hz; however,many phonemes and environmental sounds have a high proportion of theirenergy above this range making them inaudible to the implant user insome cases.

[0045] According to one aspect of the invention there is provided animproved pulsatile system for a cochlear prosthesis in which an Incomingaudio signal is concurrently presented to a speech feature extractor anda plurality of band pass filters, the pass bands of which are differentfrom one another and at least one of which is at a higher frequency thanthe normal range of the second formant or frequency peak of the speechsignal. The energy within these pass bands controls the amplitude ofelectrical stimulation of a corresponding number of fixed electrodepairs adjacent the basal end of the electrode array, thus providingadditional information about high frequency sounds at a tonotopicallyappropriate place within the cochlea. Preferably three additional bandpass filters are employed in the ranges of 2,000 to 2,800 Hz, 2,800 to4,000 Hz and 4,000 to 8,000 Hz.

[0046] The overall stimulation rate remains as F0 (fundamental frequencyor voice pitch) but, in addition, four electrical stimulation pulsesoccur for each glottal pulse, as compared with the F0F1F2 strategyheretofore used, in which only two pulses occur per voice pitch period.For voiced speech sounds, pulses representing the first and secondformant are provided along with additional stimulation pulsesrepresenting energy in the 2,000 to 2,800 Hz and the 2,800 to 4,000 Hzranges. For unvoiced phonemes, yet another pulse representing energyabove 4,000 Hz is provided while no stimulation for the first formant isprovided, since there is no energy in this frequency range. Stimulationoccurs at a random pulse rate of approximately 260 Hz, which is aboutdouble that used in earlier speech coding schemes.

[0047] According to another aspect of the invention there is provided amethod of processing an audio spectrum signal received from a microphoneto produce signals for stimulating a patient implantable tissuestimulating multi-channel electrode array adapted to be positioned in acochlea from the apical region of the cochlea to the basal region of thecochlea, said method comprising selecting a first dominant frequencypeak from said audio signal from a frequency band of between about 280Hz and about 1,000 Hz and stimulating at least one electrode in theapical region of said electrode array in accordance with the spectralinformation contained in said first peak; selecting a second dominantfrequency peak from said audio signal from a frequency band of betweenabout 800 Hz and about 4,000 Hz and stimulating at least one electrodein the basal region of said electrode array in accordance with thespectral information contained in said second peak; extracting spectralinformation in at least one region of the spectrum of said audio signaland stimulating at least one predetermined electrode in said electrodearray in accordance with said extracted spectral information, saidpredetermined electrode being in said basal region of said electrodearray.

[0048] Preferably, additional preselected electrodes are stimulatedusing spectral energy derived from said audio signal in the audiofrequency regions 2,000 to 2,800 Hz, 2,800 to 4,000 Hz and above 4,000Hz respectively.

[0049] In accordance with another aspect of the invention there isprovided an improved speech processor for a cochlear prosthesis whichemploys a multi-spectral peak (MPEAK) coding strategy to extract anumber, for example five, of spectral peaks from an incoming acousticsignal received by a microphone. The speech processor encodes thisinformation into sequential pulses that are sent to selected electrodesof a cochlear implant. The first formant (F1) spectral peak (280-1000Hz) and the second formant (F2) spectral peak (800-4000 Hz) are encodedand presented to apical and basal electrodes, respectively. F1 and F2electrode selection follows the tonotopic organization of the cochlea.High-frequency spectral information is sent to more basal electrodes andlow-frequency spectral information is sent to more apical electrodes.Spectral energy in the regions of 2000-2800 Hz, 2800-4000 Hz, and above4000 Hz is encoded and presented to three fixed electrodes. Thefundamental or voicing frequency (F0) determines the pulse rate of thestimulation during voiced periods and a pseudo-random a periodic ratedetermines the pulse rate of stimulation during unvoiced periods. Theamplitude of the acoustic signal in the five bands determines thestimulus intensity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] While the specification concludes with claims particularlypointing out and distinctly claiming the subject matter of thisinvention, it is believed that the invention will be better understoodfrom the following description, taken in conjunction with theaccompanying drawing, in which:

[0051]FIGS. 1A and 1B are interior views of the anatomy of a human earand a cross section of a cochlea, respectively;

[0052]FIG. 2 is a block diagram of the overall cochlea implant system ofthis invention;

[0053]FIG. 3 is a pictorial view of the components of this system,including the implantable parts and the parts worn by the patient;

[0054]FIGS. 3A and 3B are respective side and end elevation views of theimplantable parts of this system;

[0055]FIG. 4 is a graph of current vs. time, showing the biphasiccurrent waveform utilized in this invention;

[0056]FIG. 5 is a graph showing an example of the sequential stimulationpattern of electrode pairs for a voiced sound using the multi-peakcoding strategy of this invention;

[0057]FIG. 6 is a graph showing an example of the sequential stimulationpattern of electrode pairs for an unvoiced sound using the multi-peakcoding strategy of this invention;

[0058]FIG. 7 is a chart showing an example of the pattern of electricalstimulation for various steady-state phonemes using the multi-peakcoding strategy of this invention;

[0059]FIG. 8 is a graph showing the standard loudness growth functionfor the speech processor of this invention; and,

[0060]FIG. 9 is a block diagram of the microphone and speech processorportions of a pulsatile type, multi-channel cochlear implant system inaccordance with this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0061] The cochlear implant system of this invention, shown in FIG. 2,comprises several components. An electrode array 1 is implanted into thecochlea. The electrode array 1 comprises a number of rings or bands ofplatinum molded with a flexible silastic carrier. Preferably, there are32 bands of platinum in total. The distal 22 bands are activeelectrodes, and have connecting wires welded to them. The proximal 10electrode bands are used for stiffening, and to act as an aid tosurgical insertion. In a typical array, the electrode rings are about0.05 mm in thickness with a width of 0.3 mm, and have outside diametersranging from 0.6 mm at the proximal end to 0.4 mm diameter at the distalend. The diameter of the rings changes smoothly so that the array istapered over the distal 10 mm or so. The rings are spaced on 0.75 mmcenters over the distal 25 mm of the electrode array, and all of theexposed outside area of the rings is used as active electrode area. Thesilastic material may be MDX4-4210, manufactured by Dow Corning.

[0062] The 22 electrode wires pass via a cable 2 from the electrodearray 1 to the receiver-stimulator unit (RSU) 3. The invention describedis not limited to the use of this design of electrode array, and anumber of alternative electrode designs as have been described in theprior art could be used. The RSU 3 receives information and power froman external source through a tuned receiving coil 5 attached to the RSUand positioned just beneath the skin. The RSU also provides electricalstimulating pulses to the electrode array 1. The power, and data onwhich electrode to stimulate, and with what intensity, is transmittedacross the skin using an inductive link 6 operating at radiofrequencies, from an external multipeak speech processor (MSP) 7. Innormal operation, the MSP picks up acoustic stimuli from a microphone 8conveniently worn, and extracts from the signal, information which isused to determine stimulation electrode, rate and amplitude.

[0063] Because each patient's response to electrical stimulation isdifferent, it is necessary to configure each patient's MSP to his or herown requirements. Thus, the MSP has a random access memory (RAM) whichis programmed to suit each patient.

[0064] The patient's response to electrical stimulation is tested someshort time after implantation of the RSU 3, using the patient's MSP, andthe results of these tests are used to set up the MSP for the patient'sown particular requirements. This is done by connecting the MSP, via aconnector and cables 9, to a diagnostic programming interface unit (DPI)10. The DPI is itself connected via a cable and connector 11 to ageneral purpose computer referred to as a diagnostic and programmingunit (DPU) 12.

[0065] A pictorial representation of the system used by the patient isshown in FIGS. 3, 3A and 3B. The electrode array 20 is flexible and fitsthe shape of the cochlea (FIGS. 1A and 1B) as it is inserted along thebasilar membrane separating the scala tympani from the remainder of thecochlea. The electrode array is connected via a silastic-covered cable21 to the RSU 22. Cable 21 is specially designed to provide stressrelief to prevent fracture of the wire in the cable. The receiving coilfor information and power is a single turn of multistrand platinum wire23 which is transformer coupled to the implanted electronics in the RSU22.

[0066] An externally worn transmitting coil 24 is held against the headover the site of the RSU implant 22 by, for example, cooperating magnets(not shown) carried adjacent each of the coils 23 and 24, or by afixture (not shown) attached to the coil 24 for holding the coil to theuser's head, or by adhesive tape. Coil 24 is connected to the speechprocessor 29 via a coil cable 26 and a hearing aid microphone 27.Hearing aid microphone 27 is worn on the ear nearest to the implant siteand audio data from the microphone 27 is connected via a three wirecable 28 to the MSP 29. Transmission data is connected to the coil 24from the MSP 29 via the same three wire cable 28 and via the coil cable26. This three wire arrangement is described in the copending U.S.patent application No. 404,230, filed Sep. 7, 1989, of Christopher N.Daly, entitled “Three Wire System For Cochlear Implant Processor,” whichapplication is assigned to the assignee of the present invention and isincorporated herein by reference. Alternative microphone configurationsare possible, including a microphone worn on a tie clasp or attached tothe users clothing, or the like.

[0067] The coil cable 26 and three wire cable 28 are attached to themicrophone 27 and MSP 29 by demountable connectors 32, 33 and 34. TheMSP 29 is powered by conventionally available batteries (e.g., a singleAA size cell) carried inside the MSP 29. A plug-in jack 31 is providedto allow connection of external audio signal sources, such as from atelevision, radio, or high quality microphone.

[0068] Referring to FIG. 4, the pulse which is used to electricallystimulate the cochlea is biphasic. That is, it comprises a period ofnegative current stimulation, followed by an equal period of positivecurrent stimulation of equal amplitude, the two periods (known as phasesphi 1 and phi 2), separated by a short period of no stimulation. Phi 1and phi 2 may be in the range of 12 to 400 microseconds (typically 200microseconds), and the intervening interval is typically about 50microseconds. The amplitudes of phi 1 and phi 2, their durations, andthe duration of the intervening interval are determined by theinformation decoded from the signal transmitted by the speech processor29 (FIG. 3). The actual values of these parameters will be set up on anelectrode by electrode basis, for each patient, as a result ofpsychophysical testing of the patient. The reversal in polarity of phi 1and phi 2 is important since it ensures that there is no net DCcomponent in the stimulus. This is important because long term DCexcitation might cause electrode corrosion, and possible subsequentdamage to the cochlea itself.

[0069] The questions of electrode electrochemistry and charge balanceare thought to be more important in cochlear implants than in, say,cardiac pacemakers which are well known in the art. This is because acochlear stimulator will be stimulating nerve fibers, whereas a cardiacpacemaker is designed to stimulate cardiac muscle. It is thought thatnerve tissues may be more susceptible to damage due to electricalstimulation, and thus the cochlear implant system is designed with morestringent safety factors than cardiac pacemakers. The system is designedso that the same stimulus source is used for both stimulation phases.The biphasic pulse is produced simply by reversal of the connections tothe electrodes. Thus, extremely good charge symmetry is obtained,resulting in a high level of safety, provided the durations of phi 1 andphi 2 are equal.

[0070] The stimulation circuitry is preferably configured as a constantcurrent source. This has the advantage compared to a constant voltagesource that if the electrode impedance changes (as has often beenobserved) the delivered current to the electrode will remain unalteredover a large range of electrode impedances. The current may be variedfrom a few microamps to 2 mA, allowing a very large range of loudnesspercepts to be produced and large variations between patients to beaccommodated.

[0071] The stimulus generation circuitry in the RSU 3 (FIG. 2) ispreferably designed to operate in one of two modes. The first mode isreferred to as “multipolar” or “common ground” stimulation. In thismode, one electrode is selected to be the “active” electrode, and allother electrodes operate as a common current source. In phase phi 2, theconnections are reversed so that the “active” electrode acts as thecurrent source and the common electrodes act as a current sink. Thechoice of stimulus order is not determined by any limitations orrestrictions in the circuit design, and either way may be chosen whenimplementing the circuit design.

[0072] The second mode is “bipolar” stimulation. In this modestimulation is between two selected electrodes, let us say A and B. Inphase phi 1, current is sourced by A, and sunk by B. In phase phi 2,current is sourced by B, and sunk by A, and no other electrodes play anypart in stimulation. The RSU 3 is preferably configured so that any pairof electrodes may be selected for bipolar stimulation. Thus there isgreat flexibility in choice of stimulation strategy.

[0073] It should be understood that only these two particularstimulation modes have been chosen. Other stimulation modes are notexcluded, however. For example, a multipolar or distributed groundsystem could be used where not all other electrodes act as a distributedground, and any electrode could be selected at any time to be a currentsource, current sink, or inactive during either stimulation phase withsuitable modification of the receiver-stimulator.

[0074] The main aim of this invention is to provide improved speechcommunication to those people suffering from profound hearing loss.However, in addition to providing improved speech communication, it isalso important to be able to convey environmental sounds, for exampletelephones, doors, warning sirens, door bells, etc., which form part ofa person's life. The system described up to now is basically that of theCrosby et al. patent, heretofore referred to and incorporated herein byreference. In the Crosby et al. patent it is recognized that the secondformant F2 carries most of the intelligibility of the speech signal,while the first formant F1, although containing much of the naturalnessof the signal, contributes little to intelligibility.

[0075] Crosby et al. observed that the third and higher formants do notcarry as much information as the second formant. They also felt that inview of the then limitations of knowledge on the interaction betweenelectrodes when a number of electrodes are stimulated simultaneously,the most effective method of stimulation would be to code the secondformant on an appropriate electrode or site in the cochlea to providethe most important formant information. The amplitude of suchstimulation is derived from the amplitude of the second formant.

[0076] The Crosby et al. system also provides prosodic information inthe form of pulse rate. That system compresses the stimulation rate tothe range 100-250 Hz, the range in which the greatest pitchdiscrimination from stimulation pulse rate is achieved.

[0077] An additional factor employed in Crosby et al. is that only thetop 10 to 20 dB of current acoustic stimulus level is used to determinestimulus amplitude. That is, instead of compressing the entire acousticloudness range into the small range of electrical stimulation available,only the top part is used. Thus, Crosby et al.'s amplitude of the signalis entirely represented by a five bit binary code, which provides only30 dB of dynamic range.

[0078] In summary, the Crosby et al. speech processing strategy is:

[0079] 1. The dominant spectral peak in the range of about 300 Hz toabout 4000 Hz is used to encode electrode position.

[0080] 2. The amplitude of the dominant spectral peak used to encodeelectrode position is used to determine stimulation amplitude.

[0081] 3. Voice pitch (F0) is compressed and used to determine thestimulation rate.

[0082] For unvoiced sounds and, environmental sounds, the Crosby et al.system still generates stimuli, but the stimulation rate and electrodeposition will be determined by the exact nature of the acoustic signal.For example, for sibilant consonants (“s”), the stimulation rate isfairly fast, but not constant, and the electrode stimulated will be onewhich illicits a high frequency percept.

[0083] A second speech processing strategy, useful in some patients, isemployed in Crosby et al. The second strategy is similar to the onementioned above in that electrode position is encoded from formantfrequency. However, the stimulation rate is at the F1 of first formantfrequency, and the stimulation amplitude is determined for the value ofthe peak of the acoustic signal at the time of the F1 peak. This has theadvantage that the stimulation rate is faster, and elicits more naturalsounding speech perceptions in some patients. In addition, since the F1signal is amplitude modulated and temporally better than the F0 rate,the patients also perceive the F0 or voice pitch which is useful forconveying prosodic information.

[0084] Another speech processing strategy considered in the Crosby etal. reference is to stimulate the patient at the rate of F1 extractedfrom an incoming speech signal, but to pattern the stimulation such thatthe stimuli are gated at the F0 rate.

[0085] Notwithstanding the success of speech processors using the Crosbyet al. F0, F1, F2 speech processing coding scheme over the last fewyears, a number of problems still remain in connection with the use ofsuch speech processor coding schemes. As indicated earlier, patients whoperform well in quiet conditions can have significant problems whenthere is a moderate level of background noise. Moreover, since theF0,F1,F2 scheme codes frequencies up to about 4000 Hz, and many phonemesand environmental sounds have a high proportion of their energy abovethis range, such phonemes and environmental sounds are inaudible to theimplant user in some cases.

[0086] In accordance with the present invention multichannel cochlearimplant prostheses having a pulsatile operating system, such as thatdisclosed in the Crosby et al. reference, are provided with a speechcoding scheme in which the speech signal is bandpass filtered into anumber of bands, for example 3, within and beyond the normal range ofthe second frequency peak or formant F2 of the speech signal. The speechcoding scheme disclosed herein is referred to as the multi-spectral peakcoding strategy (MPEAK). MPEAK is designed to provide additionalhigh-frequency information to aid in the perception of speech andenvironmental sounds.

[0087] The MPEAK coding strategy extracts and codes the F1 and F2spectral peaks, using the extracted frequency estimates to select a moreapical and a more basal pair of electrodes for stimulation. Eachselected electrode is stimulated at a pulse rate equal to thefundamental frequency F0. In addition to F1 and F2, three high frequencybands of spectral information are extracted. The amplitude estimatesfrom band three (2000-2800 Hz), band four (2800-4000 Hz), and band five(above 4000 Hz) are presented to fixed electrodes, for example theseventh, fourth and first electrodes, respectively, of the electrodearray 1 (FIG. 2).

[0088] The first, fourth and seventh electrodes are selected as thedefault electrodes for the high-frequency bands because they are spacedfar enough apart so that most patients will be able to discriminatebetween stimulation at these three locations. Note that these defaultassignments may be reprogrammed as required. If the three high frequencybands were assigned only to the three most basal electrodes in the MAP,many patients might not find the additional high frequency informationas useful since patients often do not demonstrate good place-pitchdiscrimination between adjacent basal electrodes. Additionally, theoverall pitch percept resulting from the electrical stimulation might betoo high.

[0089] Table I below indicates the frequency ranges of the variousformants employed in the speech coding scheme of the present invention.TABLE I Frequency Range Formant or Band  280-1000 Hz F1  800-4000 Hz F22000-2800 Hz Band 3 - Electrode 7 2800-4000 Hz Band 4 - Electrode 1 4000Hz and above Band 5 - Electrode 1

[0090] If the input signal is voiced, it has a periodic fundamentalfrequency. The electrode pairs selected from the estimates of F1, F2 andbands 3 and 4 are stimulated sequentially at the rate equal to F0. Themost basal electrode pair is stimulated first, followed by progressivelymore apical electrode pairs, as shown in FIG. 5. Band 5 is not presentedin FIG. 5 because negligible information is contained in this frequencyband for voiced sounds.

[0091] If the input energy is unvoiced, energy in the F1 band (280-1000Hz) is effectively zero. Consequently it is replaced with the frequencyband that extracts information above 4000 Hz. In this situation, theelectrodes pairs selected from the estimates of F2, and bands 3, 4 and 5receive the pulsatile stimulation. The rate of stimulation is aperiodicand varies between 200-300 Hz. FIG. 6 shows the sequential stimulationpattern for an unvoiced sound, with stimulation progressing from base toapex. The MPEAK coding strategy thus may be seen to extract and codefive spectral peaks but only four spectral peaks are encoded for any onestimulus sequence.

[0092]FIG. 7 illustrates the pattern of electrical stimulation forvarious steady state phonemes when using the MPEAK coding strategy. Aprimary function of the MAP is to translate the frequency of thedominant spectral peaks (F1 and F2) to electrode selection. To performthis function, the electrodes are numbered sequentially starting at theround window of the cochlea. Electrode 1 is the most basal electrode andelectrode 22 is the most apical in the electrode array. Stimulation ofdifferent electrodes normally results in pitch perceptions that reflectthe tonotopic organization of the cochlea. Electrode 22 elicits thelowest place-pitch percept, or the “dullest” sound. Electrode 1 elicitsthe highest place-pitch percept, or “sharpest” sound.

[0093] To allocate the frequency range for the F1 and F2 spectral peaksto the total number of electrodes, a default mapping algorithm splits upthe total number of electrodes available to use into a ratio ofapproximately 1:2, as shown in FIG. 7. Consequently, approximately onethird of the electrodes are assigned to the F1 frequency range. Theseare the more apical electrodes and they will cover the frequency rangeof 280-1000 Hz. The remaining two thirds of the electrodes are assignedto the F2 frequency range (800-4000 Hz). The most apical electrodes,which cover the frequency range from 280-1000 Hz, are assigned linearlyequal frequency bands. The frequency range corresponding to the estimateof F2 is assigned to the remaining more basal electrodes and is dividedinto logarithmically equal frequency bands. This frequency distributionis called linear/log (lin/log) spacing.

[0094] A second optional mapping algorithm (not shown) splits up thetotal frequency range into logarythmically equal frequency bands forboth F1 and F2 electrode groups (log/log spacing). In comparison to thelin/log spacing, this results in relatively broad frequency bands forelectrodes that are assigned frequency boundaries below 1000 Hz. Becauseof the wider frequency bands for these electrodes, many vowel soundswill stimulate similar electrodes, thus making discrimination of thesevowels difficult.

[0095] The F1/F2 lin/log function of the default algorithm is preferablebecause it gives better spatial resolution in the F1 range than thelog/log function. In addition, this algorithm provides discrimination ofvowels and consonants with formants close to 1000 Hz.

[0096] The mapping section of the DPS program allows flexibility inassigning frequency bands to electrodes. If fewer electrodes areincluded in the MAP, then fewer and wider frequency bands are allocatedautomatically by the computer so that the entire frequency range iscovered. Furthermore, it is possible to override the computer-generatedspacing of frequency bands. Any range of frequencies may be allocated toany electrode or electrodes by changing the upper frequency boundaries.

[0097] Table II, below, shows the default boundaries (lin/log) for a MAPcreated in the biphasic +1 mode using 20 electrode pairs and the MPEAKcoding strategy. TABLE II Lin/Log Frequency Boundaries for 20 Electrodesin a BP +1 Mode. Also Shown are the electrode allocations for the threehigh frequency bands. Frequency Boundaries Electrode Lower Upper 20  280 400 19  400  500 18  500  600 17  600  700 16  700  800 15  800  900 14 900 1000 13 1000 1112 12 1112 1237 11 1237 1377 10 1377 1531  9 15311704  8 1704 1896  7 1896 2109  6 2109 2346  5 2346 2611  4 2611 2904  32904 3231  2 3231 3595  1 3595 & above

[0098] Table III, below, shows the default boundaries in the same modeusing only 14 electrode pairs and the MPEAK coding strategy. TABLE IIILin/Log Frequency Boundaries for 14 Electrodes in a BP +1 Mode. Alsoshown are the electrode allocations for the three high frequency bands.Frequency Boundaries Electrode Lower Upper 20  280  400 18  400  550 17 550  700 16  700  850 15  850 1000 14 1000 1166 13 1166 1360 10 13601587  9 1587 1851  8 1851 2160  7 2160 2519  6 2519 2939  5 2939 3428  43428 & above

[0099] The amplitude of the electrical stimulus is determined from theamplitude of the incoming acoustic signal within each of the fivefrequency bands (F1, F2, Bands 3, 4 and 5). However, because theelectrodes have different threshold (T) and maximum acceptable loudness(C) levels, the speech processor must determine the level of stimulationfor each electrode separately based on the amplitude of the incomingsignal in each band.

[0100] The MSP (FIG. 2) contains a non-linear loudness growth algorithmthat converts acoustic signal amplitude to electrical stimulationparameters. First, the MSP converts the amplitude of the acoustic signalinto a digital linear scale with values from 0 to 150, as may be seennow by reference to FIG. 8. That digital scale (in combination with theT and C-levels stored in the patient's MAP) determines the actual chargedelivered to the electrodes. Signals whose amplitude levels are coded as1 will cause stimulation at the T-level. Signals whose amplitude levelsare coded as 150 will cause stimulation at the C-level.

[0101] Referring now to FIG. 9, a block diagram of the microphone andspeech processor portions of a pulsatile type, multi-channel cochlearimplant system 100 have there been illustrated. The system 100 includesa microphone 110 which picks up speech and provides electrical audiosignals to a speech feature extractor 112 through an automatic gaincontrol amplifier 111. The speech feature extractor 112 analyzes thesignals and provides digital outputs corresponding to the frequenciesand amplitudes of the first and second formants, identified as F1, A1,F2 and A2, respectively, in FIG. 10.

[0102] The speech feature extractor 112 also detects and outputs thevoice pitch F0 and starts the encoder 113, which translates, using a MAP114 containing information on the patient's psychophysical test results,the voice pitch information into a pattern of electrical stimulation ontwo electrodes that are stimulated sequentially. The data so translatedis sent by the patient coil 115 to the implanted receiver stimulatorunit RSU 3 (FIG. 2).

[0103] Three bandpass filters 116, 117 and 118 also receive the audiosignal from microphone 110 before it is applied to the speech featureextractor 112, and separate the signal into three components ofdifferent frequencies, a 2000-2800 Hz signal in band 3, a 2800-4000 Hzsignal in band 4, and 4000-8000 Hz signal in band 5. The signals frombands 3, 4 and 5 are lead to the encoder 113 and mapping of thesesignals is done in a manner similar to that for the first and secondformants, and translation of the resulting pattern of electricalstimulation to the appropriate electrodes takes place, as discussedearlier herein.

[0104] The automatic gain control amplifier 111 is used to control theamplitude of the signal fed to the filters 116 and 117. Since filter 118is only used for unvoiced parts of the speech signal, its amplitude isnever very great and, therefore, the signal does not require automaticgain control. Accordingly, amplifier 119 does not have automatic gaincontrol provisions incorporated therein.

[0105] To summarize, the psychophysical measurements that are made usingthe DPS software provide the information for translating the extractedacoustic input into patient-specific stimulation parameters. Threshold(T) and maximum (C) levels for electrical stimulation are measured foreach electrode pair. These values are stored in the MAP. They determinethe relationship between the incoming acoustic signal amplitude and thestimulation level for any given electrode pair.

[0106] Inside the speech processor a random access memory stores a setof number tables, referred to collectively as a MAP. The MAP determinesboth stimulus parameters for F1, F2 and bands 3-5, and the amplitudeestimates. The encoding of the stimulus parameters follows a sequence ofdistinct steps. The steps may be summarized as follows:

[0107] 1. The first formant frequency (F1) is converted to a numberbased on the dominant spectral peak in the region between 280-1000 Hz.

[0108] 2. The F1 number is used, in conjunction with one of the MAPtables, to determine the electrode to be stimulated to represent thefirst formant. The indifferent electrode is determined by the mode.

[0109] 3. The second formant frequency (F2) is converted to a numberbased on the dominant spectral peak in region between 800-4000 Hz.

[0110] 4. The F2 number is used, in conjunction with one of the MAPtables to determine the electrode to be stimulated to represent thesecond formant. The indifferent electrode is determined by the mode.

[0111] 5. The amplitude estimates for bands 3, 4 and 5 are assigned tothe three default electrodes 7, 4 and 1 for bands 3, 4 and 5,respectively, or such other electrodes that may be selected when the MAPis being prepared.

[0112] 6. The amplitude of the acoustic signal in each of the frequencybands is converted to a number ranging from 0-150. The level ofstimulation that will be delivered is determined by referring to a setMAP tables that relate acoustic amplitude (in range of 0-150) tostimulation level for the specific electrodes selected in steps 2, 4 and5, above.

[0113] 7. The data are further encoded in the speech processor andtransmitted to the receiver/stimulator. It, in turn, encodes the dataand sends the stimuli to the appropriate electrodes. Stimulus pulses arepresented at a rate equal to F0 during voiced periods and at a randomaperiodic rate within the range of F0 and F1 formants (typically 200 to300 Hz) during unvoiced periods.

[0114] It will be apparent from the foregoing description that themulti-spectral peak speech coding scheme of the present inventionprovides all of the information available in the prior art F0F1F2scheme, while providing additional information from three high frequencyband pass filters. These filters cover the following frequency ranges:2000 to 2800 Hz, 2800 to 4000 Hz and 4000 to 8000 Hz. The energy withinthese ranges controls the amplitude of electrical stimulation of threefixed electrode pairs in the basal end of the electrode array. Thus,additional information about high frequency sounds is presented at atonotopically appropriate place within the cochlea.

[0115] The overall stimulation rate remains as F0 (fundamental frequencyor voice pitch) but in the scheme of the present invention fourelectrical stimulation pulses occur for each glottal pulse. Thiscompares with the prior F0F1F2 strategy in which only two pulses occurper voice pitch period. In the new coding scheme, for voiced speechsounds, the two pulses representing the first and second formant arestill provided, and additional stimulation pulses occur representingenergy in the 2000-2800 Hz and the 2800-4000 Hz ranges.

[0116] For unvoiced phonemes, yet another pulse representing energyabove 4000 Hz is provided while no stimulation for the first formant isprovided, since there is no energy in this frequency range. Stimulationoccurs at a random pulse rate of approximately 260 Hz, which is aboutdouble that used in the earlier strategy.

[0117] It will be further apparent from the foregoing description thatthis invention provides an improved cochlear implant system whichovercomes various of the problems associated with earlier cochlearimplant systems. The use of a multi-spectral peak speech coding strategyin accordance with this invention provides the user of the implantsystem with significantly improved speech recognition, even in thepresence of moderate levels of background noise. In addition improvedrecognition of phonemes and environmental sounds are provided by thisinvention.

[0118] While a particular embodiment of this invention has been shownand described, it will be obvious to those skilled in the art thatvarious changes and modifications may be without departing from thisinvention in its broader aspects, and it is, therefore, aimed in theappended claims to cover all such changes and modifications as fallwithin the true spirit and scope of this invention.

What is claimed is:
 1. A multi-channel cochlear prosthesis including apatient implantable tissue stimulating multi-channel electrode arrayadapted to be positioned in a cochlea from the apical region of thecochlea to the basal region of the cochlea, a patient implantablemulti-channel stimulator connected to said array, and a patientexternally worn programmable speech processor for processing soundsignals into electrical stimulation signals that are transmitted to saidstimulator, said prosthesis further comprising: means based on adominant peak extraction in the region of between about 280 Hz to about1000 Hz for determining a first formant spectral information in saidsound signal and stimulating at least one electrode in the apical regionof said electrode array in accordance with the spectral information ofsaid formant; means based on a dominant spectral peak extraction in theregion of between about 800 Hz and about 4000 Hz for determining asecond formant frequency in said sound signal and stimulating at leastone electrode in said basal region of said electrode array in accordancewith the spectral information of said formant; and, at least one highfrequency band filter for extracting spectral information in at leastone region of the spectrum of said sound signal and stimulating at leastone predetermined electrode in said electrode array in accordance withsaid extracted spectral information, said predetermined electrode beingin said basal region of said electrode array.
 2. A multi-channelcochlear prosthesis according to claim 1, including a plurality of saidhigh frequency band filters for extracting spectral information in acorresponding number of regions of said sound signal and stimulating atleast a corresponding number of said predetermined electrodes in saidelectrode array, all of said predetermined electrodes being in saidbasal region of said electrode array.
 3. A multi-channel cochlearprosthesis according to claim 2, wherein said electrical stimulationsignals are applied to said electrodes in the form of pulses presentedat a pulse rate dependent on the pitch of the sound signal.
 4. Amulti-channel cochlear prosthesis according to claim 3, wherein saidpulse rate is in the range of between about 80 Hz and about 400 Hz.
 5. Amulti-channel cochlear prosthesis according to any one of claims 1-4including at least three of said high frequency band filters each withpredetermined electrodes.
 6. A multi-channel cochlear prosthesisaccording to claim 5, wherein a first one of said high frequency bandfilters extracts spectral information from the sound signal in afrequency range of between about 2000 Hz and about 2800 Hz, wherein asecond one of said high frequency band filters extracts spectralinformation from the sound signal in a frequency range of between about2000 Hz and about 2800 Hz, and wherein a third one of said highfrequency band filters extracts spectral information from the soundsignal in a frequency range of between about 4000 Hz and about 8000 Hz.7. A multi-channel cochlear prosthesis according to claim 6, wherein theelectrodes in said electrode array may be considered to be consecutivelynumbered, starting from the basal end thereof and extending to theapical end thereof, and wherein amplitude estimates derived from thespectral information extracted from said first, second and third highfrequency band filters are applied to said corresponding number ofpredetermined electrodes with the amplitude estimate from said firstfilter being applied to a higher-numbered electrode than the amplitudeestimate from said second filter and the amplitude estimate from saidsecond filter being applied to a higher-numbered electrode than theamplitude estimate from said third filter.
 8. A multi-channel cochlearprosthesis according to claim 7, wherein said electrode array includesabout 22 electrodes therein, wherein said basal region of said electrodearray comprises about two-thirds of the electrodes in said electrodearray, and wherein said apical region comprises about one-third of theelectrodes in said electrode array.
 9. A multi-channel cochlearprosthesis according to claim 8, wherein said amplitude estimatesderived from the spectral information extracted from said first, secondand third high frequency band filters are applied to said seventh,fourth and first electrodes, respectively, in said electrode array. 10.A multi-channel cochlear prosthesis according to claim 7, wherein, inthe case of voiced sound signals, the electrodes selected to bestimulated are based on the first and second formants and on informationderived from the first and second filters, and wherein said electrodesare stimulated sequentially at a rate that is based on the pitch of thesound signal, with the most basal of said electrodes being stimulatedfirst, followed by stimulation of progressively more apical electrodes.11. A multi-channel cochlear prosthesis according to claim 7, wherein,in the case of unvoiced sound signals, the electrodes selected to bestimulated are based on the second formant and on information derivedfrom the first, second and third filters, and wherein said electrodesare stimulated sequentially at an aperiodic rate within the range offrom formant F0 to formant F1, with the most basal of said electrodesbeing stimulated first, followed by stimulation of progressively moreapical electrodes.
 12. A multi-channel cochlear prosthesis according toclaim 7, wherein said electrodes are stimulated at an aperiodic ratewithin the range of about 200 Hz to about 300 Hz.
 13. A multi-channelcochlear prosthesis according to claim 11, wherein said aperiodic rateis within the range of about 200 Hz to about 300 Hz.
 14. A method ofprocessing an audio spectrum signal received from a microphone toproduce signals for stimulating a patient implantable tissue stimulatingmulti-channel electrode array adapted to be positioned in a cochlea fromthe apical region of the cochlea to the basal region of the cochlea,said method comprising selecting a first dominant frequency peak fromsaid audio signal from a frequency band of between about 280 Hz andabout 1,000 Hz and stimulating at least one electrode in the apicalregion of said electrode array in accordance with the spectralinformation contained in said first peak; selecting a second dominantfrequency peak from said audio signal from a frequency band of betweenabout 800 Hz and about 4,000 Hz and stimulating at least one electrodein the basal region of said electrode array in accordance with thespectral information contained in said second peak; extracting spectralinformation in at least one region of the spectrum of said audio signaland stimulating at least one predetermined electrode in said electrodearray in accordance with said extracted spectral information, saidpredetermined electrode being in said basal region of said electrodearray.
 15. A method of processing an audio spectrum signal as claimed inclaim 14 wherein additional preselected electrodes are stimulated usingspectral energy derived from said audio signal in the audio frequencyregions 2,000 to 2,806 Hz, 2,800 to 4,000 Hz and above 4,000 Hzrespectively.
 16. (new) A method of speech coding an audio spectrumsignal received from a microphone to produce signals for stimulating apatient implantable tissue stimulating multi-channel electrode arrayadapted to be positioned in a cochlea from the apical region of thecochlea to the basal region of the cochlea, said method comprisingbandpass filtering said audio spectrum signal into a plurality of bandswithin and beyond the normal range of a second (F2) formant frequencypeak of said audio spectrum signal whereby additional high frequencyinformation is provided to a patient.
 17. (new) The method of claim 16wherein information thus derived from said audio spectrum signal isencoded into sequential pulses and applied to selected electrodes ofsaid electrode array.